水力压裂裂缝穿层及扭转扩展的三维模拟分析

 应用并行有限元程序对水力压裂过程进行真三维数值模拟，实现对压裂裂缝起裂、扩展及扩展中的穿层、扭转行为的全过程分析；数值计算中无需假定压裂裂缝的起裂位置和扩展路径，即可根据实际岩体水力学模型的力学、水力学等边界条件，自动标定出压裂裂缝的三维扩展模式，并显示出该并行有限元程序对复杂地质力学条件下水力压裂过程三维模拟分析的适用性。通过对压裂裂缝扩展过程中孔隙压力分布、裂缝几何形状和尺寸的演化进行了解读，显示出压裂裂缝的扩展模式与地层分布密切相关。当生产层很薄或生产层与上下阻挡层岩性差别较大时，裂缝穿层现象突出，可能会出现压裂实际缝长远远小于设计缝长的现象；近场地应力差异、地层分布特征及岩体细观非均匀性都有可能诱发压裂裂缝扭转扩张。分析结果对水力压裂施工设计具有一定的参考价值。

THREE-DIMENSIONAL NUMERICAL ANALYSIS OF TRAVERSING AND TWISTED FRACTURES IN HYDRAULIC FRACTURING

Numerical tests with parallel finite element method(FEM) are conducted to investigate the hydraulic fracturing. The fracture initiation，propagation and associated traversing，twisted fractures are numerically obtained，which indicate that the finite element model has potential applications to a larger range of complicated hydraulic fracturing problems. As the proposed method is used to conduct the numerical simulation of hydraulic fracturing，the three-dimensional fractures can be achieved directly without any artificial assumption on the initiation and propagation of fractures. Numerical results，including the pore pressure field，fracture shape and size，show that the mechanical parameters of strata have an important influence on the fracture mode. The traversing fractures are prone to emergence if the producing zone is thin or the difference of rock mechanical parameters is relatively great. The traversing fractures are prone to induce the failure of hydraulic fracturing. The twisted fractures are mainly caused by in-situ stress distribution，stratum characteristic and the heterogeneous characteristics of rock at mesoscopic level. The investigation can provide significantly meaningful guides for the design of hydraulic fracture engineering in practice.